Since a foamed body has the property of absorbing vibration, it has been used widely in the application as a sound-proofing material, a buffer material or a stress-relaxing material. As described in Japanese Unexamined Patent Publication (Kokai) No. 9-78038 and WO99/03943, the foamed body is sometimes used as a substrate of an adhesive tape or an adhesive sheet (hereinafter called the “adhesive tape, etc”), since the foamed body is superior in flexibility, and is therefore easily conformable to an adherend.
Specifically, Japanese Unexamined Patent Publication (Kokai) No. 9-78038 uses, as a substrate material, a foamed body using an elastomer such as epichlorohydrin rubber or an ethylene-propylene-diene terpolymer (EPDM) for a matrix material. However, elastomers are generally difficult-to-bond materials, and a pressure sensitive adhesive cannot be easily applied by coating or lamination. The foamed body of the epichlorohydrin rubber, in particular, contains chlorine, and a careful attention must be paid when this foamed body is discarded. The foamed body of EPDM contains large quantities of process oil. This oil is likely to bleed from the EPDM foamed body and is not much desirable for use in the pressure sensitive adhesive.
On the other hand, a foamed pressure sensitive adhesive, in which the pressure sensitive adhesive itself is a foamed body, is known, too.
For example, the WO99/03943 specification describes a foamable double-sided pressure sensitive adhesive tape produced by dispersing and packing a plurality of microcapsules in a cross-linkable tacky matrix material. According to the invention of WO99/03943, the microcapsules can impart the compression restoration force to the double-sided pressure sensitive adhesive tape, but are likely to restrict the materials of the adhesive. As a result, the adhesive tape has a low stress relaxing property and can rarely reduce the compressive load.
Japanese Unexamined Patent Publication (Kokai) No. 63-225684 describes a foamed pressure sensitive adhesive layer having both cross-linked structure and foamed structure from the aspect of improvements in heat resistance, aggregation force and stress relaxing property. More particularly, according to this Japanese Unexamined Patent Publication (Kokai) No. 63-225684, an acrylic polymer having an epoxy group (hereinafter called also as the “glycidyl group”) is treated with ultraviolet rays in the presence of a diazonium salt compound to form simultaneously the cross-linked structure and the foamed structure in the foamed pressure sensitive adhesive layer. However, the acrylic polymer or other components must be selected carefully so as not to inhibit transmission of the ultraviolet rays. If the components make it difficult for the ultraviolet rays to transmit because of their black color etc., the degree of cross-linking and foaming of the acrylic polymer becomes insufficient.
Electron beams may be used in place of the ultraviolet rays. However, the electron beams cannot transmit easily ordinary materials. Therefore, cross-linking by means of the electron beams may result in the limitation of the thickness or the degree of cross-linkage.
Japanese Unexamined Patent Publication (Kokai) No. 55-90525 discloses a pressure sensitive adhesive foamed body that is foamed and cross-linked by heat-treatment in place of the UV treatment. More particularly, a starting mixture containing an acrylic type low molecular weight copolymer having reactivity with isocyanate, polyisocyanate and a foaming agent is foamed and cross-linked by heat to give a pressure-sensitive adhesive foamed body. This reference describes that the acrylic type low molecular weight copolymer has an average molecular weight of not greater than 10,000. When the acrylic polymer used has a relatively low molecular weight of not greater than 10,000, the aggregation force of the foam cannot be obtained sufficiently because the molecular chains are short. In other word, the foamed body has high tackiness on the surface and is likely to result in an aggressive pressure sensitive adhesive, and its bonding power (from normal temperature to low temperature) is as high as that of ordinary acrylic type pressure sensitive adhesive. Therefore, this foamed body involves the problem that when it is cut, it adheres to a cutting blade and the cutting work becomes difficult. Since this foamed body uses the low molecular weight copolymer, the foam is brittle and lacks sufficient tenacity. Furthermore, since it uses the low molecular weight copolymer, the viscosity is so low that a foam having a large thickness cannot be obtained.